GB684887A - Improvements in or relating to electromagnetic waveguides - Google Patents

Abstract

684,887. Wave-guides. STANDARD TELEPHONES & CABLES, Ltd. Jan. 12, 1951, No. 952/51. Class 40 (viii). A discontinuity in a wave-guide, the effect of which is equivalent to the insertion in the otherwise reflectionless guide at a given point of either a series loading of reactance jX, or of a shunt loading of susceptance jB, is compensated by the insertion at the given point of a component equivalent to a shunt susceptance jX/Z<2>0 or a series reactance jB/Y<2>0 where Z0=1/Y0 is the characteristic impedance of the guide at the given point. The compensation can be shown to be effective over a relatively wide frequency band if the reactance is small compared to the characteristic impedance of the guide. For each discontinuity a point is found, by measurement of voltage standing wave ratio, where a negative reactive element effectively acts, and this is compensated by a positive element. For a chamfered H-type corner, Fig. 1, it can be shown from measurements of standing wave ratio at various frequencies at one open end of the corner that the residual discontinuity is equivalent to the effect of a series resonant circuit or a parallel resonant circuit of negative elements at the angle 7. These are compensated by a diaphragm 8 which provides a shunt inductance and a screw 9 which provides a shunt capacitance. The diaphragm 8 may be omitted and the requisite amount of shunt inductance provided by increasing the depth of the outer angle. In an E-type corner, Fig. 4, the perpendicular distance d between the chamfered corner 15 and the inner corner 17 is made slightly larger than the distance which would be chosen to obtain maximum standing wave ratio at the centre of the frequency band for which the corner is designed. The increase in the distance d is equivalent to the addition of a small negative capacitance in shunt which is neutralized by adjustment of a screw 16. The screws 9 and 16 are locked in position after adjustment. The effect of a mica window 20, Fig. 6, is to shunt a small capacitance across the guide and this capacitance behaves as though a small negative inductance had been connected in series between the two sections of the guide. This can be neutralized by an equal positive inductance provided by an annular cavity 23 between the flanges 21, 22 of the two sections of the guide 18, 19. The cavity need not extend completely around the circumference of the guide. Fig. 9 shows a H-type corner between a guide 31 and a horn 35 feeding to an aerial (not shown). The distance between the angle of the H-type corner and the throat of the horn (the taper, not indicated, starts at the line 41) is chosen in such a manner that the effective parallel resonant circuit of negative elements produced by the corner appears at the centre of the corner. By adjusting the distance between the face 40 of block 36 and the inner angle 52, and by adjusting screw 54, positive compensating elements are introduced, which are effective over a wide frequency band. The impedance of the horn mouth is matched to free space at mid-band frequency by an inductive diaphragm consisting of blocks 50, 51 and further compensation at mid-frequency effected by providing a window 49 at #/4 from the mouth of the horn. Compensation over a wide frequency band for the discontinuity at the start of the tapered portion is effected by metal strips 47, 48 as described in Specification 657,607. Specification 675,245, [Group XL (c)], also is referred to.